This invention relates to a rotary hearth furnace and a process of reducing a feed material in a rotary hearth furnace. More particularly, this invention relates to a rotary hearth furnace having an improved flue system and a process of reducing a feed material in a rotary hearth furnace.
A typical rotary hearth furnace includes an annular inner refractory wall, an annular outer refractory wall and an annular hearth disposed between the inner and outer walls. The hearth is movably supported on an array of rollers about its circumference. Disposed between the inner and outer walls and above the hearth is a stationary roof. A plurality of burners are positioned along the inner and/or outer walls and fire into the annular space above the rotating hearth within the stationary roof to heat a feed material that is typically conveyed on the rotating hearth through various zones, e.g., loading zone, process zone and discharge zone.
In operation, the feed material to be heated is placed directly on the hearth in the loading zone and then conveyed through the process zone wherein the feed material is subjected to radiant heating and process gases conducive to chemical reaction as the feed material is conveyed around the hearth path. The processed feed material is then removed from the rotating hearth in the discharge zone.
In a rotary hearth furnace, such as shown in U.S. Pat. Nos. 4,597,564 and 4,622,905, incorporated herein by reference in their entirety, gases that are produced in the rotary hearth furnace are exhausted from a flue positioned adjacent the loading zone and away from the discharge zone of the furnace. The flue is positioned adjacent the loading zone and away from the discharge zone of the furnace to create a counter flow system drawing gases from the discharge zone to the loading zone, i.e., the effluent flows counter to the direction of rotation of the hearth for maximum exposure time to the feed material to be heated. Although the flue systems have performed satisfactorily, it has been found that the gases produced in a typical counter flow type rotary hearth furnace near the discharge zone tend to short circuit the intended gas flow through the water seal tunnel and flow directly to the flue, thereby avoiding the process zone and the loading zone. Furthermore, it will also be appreciated that when the flue is positioned adjacent or proximate either the loading zone or the discharge zone a pressure differential is obtained at the loading zone and the discharge zone conducive to the escape of the furnace gases from the rotary hearth furnace through the loading zone and the discharge zone.
Furthermore, in the direct reduction of iron process, a high CO/CO2 ratio in the last zone of the furnace is required to prevent back oxidation of the direct reduced iron (DRI). It will be appreciated that to maintain high CO/CO2, ratios, the burners must be operated and relatively low air to fuel ratios (less than 6.2 to 1). These low air to fuel ratios translate to unacceptably low available heat values. i.e., 84.5 BTU/ft3 of natural gas at an air/fuel ratio of 6.12, which translates into high fuel usage. Three countermeasures are available to improve the available heat problem. The first is to recover energy from the furnace exhaust gases to preheat combustion air, the second is to replace some or all of the combustion air with oxygen, and the third is to combine preheated combustion air with oxygen enrichment. Table 1 summarizes the effect of preheating combustion air and oxygen enrichment on the available heat, and on the pounds of natural gas consumed per pound of direct reduced iron (DRI) produced.
In view of the foregoing, it will be appreciated that there is a significant need for an improved rotary hearth furnace that is directed to problems of the prior art. It is an object of the present invention to provide a rotary hearth furnace having improved process gas flow. It is another object of the present invention to provide a rotary hearth furnace that prevents the process gas from short circuiting the process zone through either the loading zone and/or the discharge zone of the furnace. Another object of the present invention is to provide a rotary hearth furnace that efficiently utilizes the available energy to reduce the feed material in a rotary hearth furnace. Yet another object of the present invention is to provide a rotary hearth furnace that efficiently reduces the amount of stack gases exiting the flue of a rotary hearth furnace such that the size of the exhaust equipment may be reduced. Another object of the present invention is to provide a draft away from the loading zone to eliminate release of potentially toxic vapors arising from the organic or carbonaceous binders used in preparation of the feed material through the slots of the feeder. It will be appreciated that this allows the process gas to be combined with oxygen from air and to burn thereby releasing heat within the preheat zone of the furnace. Yet another object of the present invention is to provide a rotary hearth furnace that is simple and economical to manufacture.
Briefly, according to this invention there is provided a rotary hearth furnace for reducing a feed material. The rotary hearth furnace includes a rotating hearth disposed in a refractory lined enclosure and mounted for rotary movement. The enclosure includes an annular inner wall, an annular outer wall and a roof. The enclosure is operatively sealed to the hearth and divided into a plurality of zones including at least a loading zone, a preheat zone, a process zone and a discharge zone. The furnace further includes a plurality of burners positioned in at least the outer wall of the enclosure to provide a controlled temperature within the rotary hearth furnace and a flue positioned between the preheat zone and the process zone of the furnace to exhaust combustion gases from the burners and process gases resulting from the processing of the feed material.